Electrophotographic developing methods develop by adhering toner particles in a developer to an electrostatic latent image which is formed on a photoreceptor. The developer used in such methods can be classified as either being a two-component developer composed of toner particles and carrier particles, or a one-component developer which only uses toner particles.
Among such developers, as the developing method using a two-component developer composed of toner particles and carrier particles, a cascade method or the like has long been employed. However, currently magnetic brush methods using a magnet roll have become mainstream.
In a two-component developer, carrier particles act as a carrying substance for imparting the desired charge to the toner particles and transporting the thus-imparted toner particles with a charge to the surface of the photoreceptor to form a toner image on the photoreceptor by stirring the carrier particles with the toner particles in a developing box which is filled with the developer. Carrier particles remaining on the developing roll which supports the magnets return back into the developing box from this developing roll, and are then mixed and stirred with new toner particles for reuse over a certain time period.
Unlike one-component developers, in two-component developers the carrier particles are mixed and stirred with the toner particles to charge the toner particles. The carrier particles also have a transporting function and are easily controlled when designing the developer. Therefore, two-component developers are suitable for full color developing apparatuses in which high image quality is demanded and for apparatuses performing high-speed printing in which the reliability and durability of image sustainability are demanded.
In two-component developers which are used in such a manner, the image properties, such as image density, fogging, white spots, gradation and resolution, need to exhibit a certain value from the initial stage. Furthermore, these properties must not change during printing and have to be stably maintained. To stably maintain these properties, it is necessary for the properties of the carrier particles in the two-component developer to be stable.
Conventionally, an iron powder carrier, such as iron powder covered with an oxide coating on its surface or iron powder coated with a resin on its surface, has been used for the carrier particles forming a two-component developer. These iron powder carriers have high magnetization as well as high conductance, and thus have the advantage that an image with good reproducibility of the solid portions can be easily obtained.
However, the true specific gravity of such an iron powder carrier is about 7.8, which is heavy, and its magnetization is too high. As a consequence, the toner constituent component tends to fuse to the surface of the iron powder carrier, so-called “toner spent”, from the stirring and mixing with the toner particles in the developing box. Due to the occurrence of toner spent, the effective carrier surface area decreases, whereby the frictional chargeability with the toner particles tends to deteriorate.
With a resin-coated iron powder carrier, the resin on the surface may peel away due to stress during use, causing charge to leak as a result of the high conductance, low breakdown voltage core material (iron powder) being exposed. The electrostatic latent image formed on the photoreceptor breaks down as a result of such charge leakage, thus causing brush strokes or the like to occur on the solid portions, which makes it difficult to obtain a uniform image. For these reasons, iron powder carriers, such as an oxide-coated iron powder or a resin-coated iron powder, are currently no longer used.
Recently, instead of iron powder carriers, often used are resin-coated ferrite carriers which use a ferrite having a light true specific gravity of about 5.0 and a low magnetization for the carrier and are further coated with a resin on their surface, whereby developer life has become dramatically longer.
There have been many attempts to improve the carrier properties and developer properties of such resin-coated carriers by specifying the surface properties and the like of the resin-coated carrier or carrier core material. Japanese Patent Laid-Open No. 2006-91439 describes a carrier for an electrophotographic developer with a specified shape factor (SF-1) which has excellent fluidity and is capable of forming a uniform mixed state between the toner and the carrier in a short time. In Japanese Patent Laid-Open No. 2006-91439, the shape factor (SF-1) is defined in a broad range, which means that the sphericity of the carrier also has a broad range and that variation is also large. Further, there is no description in Japanese Patent Laid-Open No. 2006-91439 regarding the BET specific surface area or apparent density of the carrier. Nevertheless, while Japanese Patent Laid-Open No. 2006-91439 does describe that the average particle size of the carrier is also 30 to 100 μm, the working examples only use a comparatively large particle size of 85 to 90 μm, and do not use small particle size carriers.
Further, Japanese Patent Laid-Open No. 2006-38961 describes a carrier for an electrophotographic developer in which a resin is coated onto a core material having a sphericity at or below a certain level and a surface roughness within a specified range. In Japanese Patent Laid-Open No. 2006-38961, by using the above-described carrier in a developer, a high-quality image is formed and image formation is stabilized against deterioration over time and environmental change. However, Japanese Patent Laid-Open No. 2006-38961 only defines sphericity and surface roughness, and does not define the variation in sphericity. Accordingly, the amount of deformed particles present is unclear. Further, there is no definition of the BET specific surface area or apparent density of the core material.
Further, Japanese Patent Laid-Open No. 9-305026 describes an image forming method using a carrier for an electrostatic latent image developer which has a shape factor (SF-1) and a shape factor (SF-2) in a specific range, in which the mechanical pressure applied to the developer is reduced so as to hardly damage the developer so that the initial properties of the developer are stably maintained even in the case of repeatedly using the developer. Further, Japanese Patent Laid-Open No. 9-319161 describes a carrier for an electrostatic latent image developer (claims 1 and 8) comprising on a core material which has a shape factor (SF-1) and a shape factor (SF-2) in a specific range a coated resin layer in which thermosetting resin microparticles having a specific critical surface tension are dispersed in a matrix resin. It is described that this carrier has a very long life, and is capable of long-term prevention of spent caused by toner on the carrier surface. Although Japanese Patent Laid-Open Nos. 9-305026 and 9-319161 do define carrier shape factor (SF-1) and shape factor (SF-2), there is no description concerning the microstructure of the surface of the carrier. Simply defining the shape factor such as in Japanese Patent Laid-Open Nos. 9-305026 and 9-319161 cannot be expected to improve startup properties when used in a developer. Further, there is no definition in Japanese Patent Laid-Open Nos. 9-305026 and 9-319161 of the BET specific surface area or apparent density of the core material.
Japanese Patent Laid-Open No. 10-39549 describes a magnetic coated carrier wherein the surface of carrier core particles having a specific resistance in a certain range or higher are coated using a resin composition at least containing a straight silicone resin and a coupling agent. This document describes that the carrier is well conferred with frictional charge with respect to the toner and that carrier adherence does not easily occur. However, Japanese Patent Laid-Open No. 10-39549 only defines the specific resistance of the carrier core particles, and contains no description concerning the BET specific surface area or apparent density. As described in Japanese Patent Laid-Open No. 10-39549, simply defining specific resistance would not be expected to improve startup properties when used in a developer.
Japanese Patent Laid-Open No. 2000-172019 describes a resin-coated carrier for a two-component developer in which the 50% particle size (D50), % by number of carrier particles smaller than 22 μm and those 62 μm or larger, and the difference between the BET specific surface area of the carrier core material after resin removal and the BET specific surface area of the resin-coated carrier are within a certain range, and in which the shape factor (SF-1) and shape factor (SF-2) are in a specific range. This document describes that when used in a developer, a stable charge amount can be maintained even if used continuously for a long time, whereby a stable copy of an image can be obtained having no reduction in image density or occurrence of blurring. However, the BET specific surface area described in Japanese Patent Laid-Open No. 2000-172019 is the BET specific surface area of the carrier core material after resin removal, and is not the BET specific surface area of the carrier core material itself, and further, there is no description concerning the apparent density. Thus, an improvement in startup properties when used in a developer cannot be expected with a resin-coated carrier such as that described in Japanese Patent Laid-Open No. 2000-172019.
As described above, a resin-coated ferrite carrier for an electrophotographic developer having good sphericity and average particle size and which has improved startup properties when used as a developer is yet to be found.